Gear Puzzle Games and How a Watch Gear Train Really Works

People who open the display back of their first mechanical watch usually tell me the same thing: it is beautiful, and they have no idea what any of it is doing. They can see a wheel spinning quickly, another one barely moving, and a gold balance swinging back and forth. Ask which wheel drives the seconds hand and almost nobody can point to it.

Oddly, many of those same people have already solved dozens of gear puzzles in video games. They know that two meshed gears turn in opposite directions and that a small gear driving a big one slows things down. What they have never done is connect that knowledge to the movement on their wrist. This piece tries to build that bridge, using the gear puzzles you have probably already played and a bit of honest arithmetic.

What gear puzzles already taught you

Cogs, a 2009 puzzle game from Lazy 8 Studios, asks you to slide tiles around a board until a chain of gears connects a source to a target. Machinarium from Amanita Design is full of little mechanical locks and contraptions. The Room series by Fireproof Games hides gear trains inside puzzle boxes, and the Professor Layton games sprinkle in brainteasers about which way the last gear in a chain will spin.

Played together, they drill three rules into you. Neighbouring gears turn in opposite directions. A small gear driving a larger one makes the larger one turn more slowly but with more force. And an idler gear placed between two others changes the direction of the last one without changing its speed.

Those three rules cover a surprising amount of a watch movement. The piece games almost never show you is the compound gear: a large wheel and a small pinion fixed to the same axle, turning together. Watchmakers call that axle an arbor, the small gear a pinion, and the teeth on a pinion leaves. Nearly every wheel in a movement is half of one of these pairs, and that single idea is what lets a watch turn a slow spring into a fast, precise tick.

The going train, wheel by wheel

The chain of wheels between the mainspring and the escapement is called the going train. If you have read our walkthrough of how a mechanical watch actually works, this is the part in the middle. Here is what each stage does in a typical movement.

The mainspring lives coiled inside the barrel, a toothed drum. The barrel turns very slowly, only a handful of times over the whole power reserve. It drives the centre wheel, which in many traditional layouts turns exactly once per hour and carries the minute hand. The centre wheel drives the third wheel, which exists mostly to step the speed up. The third wheel drives the fourth wheel, which turns once per minute, and that is the one carrying the seconds hand in a classic small-seconds or central-seconds layout. Finally, the fourth wheel drives the escape wheel, which feeds tiny pulses to the pallet fork and the balance.

The hour hand sits on a separate little set of gears called the motion works, which takes the minute hand's rotation and reduces it by exactly twelve. Puzzle players will recognise that one immediately: it is a plain reduction, small gear driving large gear, twice over.

Doing the arithmetic yourself

The satisfying part of gear puzzles is that the numbers always have to work out. Watches are the same, and you can check them with nothing more than multiplication.

Start with the step from centre wheel to fourth wheel. It must speed up by a factor of sixty, from one turn an hour to one turn a minute. One combination that works: a centre wheel with 80 teeth drives a third pinion with 10 leaves, giving 8 times. The third wheel, with 75 teeth, drives a fourth pinion with 10 leaves, giving 7.5 times. Multiply 8 by 7.5 and you get exactly 60. Real calibres use various tooth counts, but they always have to land on that number.

Now the step that decides the beat rate. A common Swiss lever escape wheel has 15 teeth, and each tooth produces two beats as it passes the pallets. Take a movement running at 28,800 vibrations per hour, the rate used in many modern calibres such as the ETA 2824. That works out to eight beats a second, or 480 a minute. Divide 480 by 30 beats per escape wheel turn, and the escape wheel has to spin 16 times for every single turn of the fourth wheel.

Do the same for a slower 21,600 vibrations per hour movement, the rate of many Seiko automatics, and you get 360 beats a minute, so the escape wheel turns 12 times per minute. A Zenith El Primero at 36,000 needs 20. Once you see it this way, the beat rate stops being a spec sheet number and becomes a gear ratio you could almost draw.

A watch is a gear puzzle with one extra rule: the last gear is not allowed to spin freely. It has to be stopped and released, a few times every second.

Where the game picture breaks down

I would not want anyone to think that solving Cogs prepares you to service a movement. Games treat gears as frictionless and perfect. Real ones are not, and most of what watchmaking is about lives in that gap.

First, a watch gear train is not really pushing toward a target the way a puzzle chain is. It is being held back. Without the escapement, the mainspring would dump all its energy in a second or two and the hands would whirl. The pallet fork lets the escape wheel advance one tooth at a time, and the balance spring sets the pace. A gear puzzle never asks you to design something that releases energy slowly on purpose.

Second, friction and wear matter enormously. Pivots run in jewel bearings, usually synthetic ruby, because steel against brass would wear quickly at these speeds. Oils age and thicken, which is a large part of why movements need service. None of that shows up in a game where gears spin forever once connected.

Third, the force passing through the train is tiny and changes as the mainspring unwinds. That is why the same watch can run a few seconds a day differently at full wind and near the end of its power reserve, a point covered in what a power reserve actually tells you. Puzzle gears have no such mood swings.

Seeing it in your own watch

If your watch has a display back, there is a quick way to put all this to use. Lay the watch face down under a good lamp and watch the movement for one full minute. The escape wheel is the small, fast one near the balance, flicking forward in tiny jumps. The fourth wheel is harder to spot because many movements hide it under a bridge, but if your seconds hand sits on its own arbor you can often see its pinion turning once in that minute.

Then look up your calibre's beat rate and do the escape wheel division from the section above. Count the ticks you hear in ten seconds if you can: about 80 means 28,800, about 60 means 21,600. It takes five minutes and, in my experience, changes the way people look at that spinning metal for good. For the parts beyond the gear train, what a movement does inside the case is the natural next read, and more crossovers between games and watches live in the games section.